With the diminishing supply of crude mineral oil, use of renewable energy sources is becoming increasingly important for the production of fuels and chemicals. These fuels and chemicals from renewable energy sources are often referred to as biofuels, respectively biochemicals.
Biofuels and/or biochemicals derived from non-edible renewable energy sources, such as lignocellulosic material, are preferred as these do not compete with food production. These biofuels and/or biochemicals are also referred to as second generation or advanced biofuels and/or biochemicals.
D. Humbrid et al in their report titled “Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol—Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover”, published by the National Renewable Energy Laboratory (NREL) as technical report NREL/TP-5100-47764 in May 2011 describe a process using co-current dilute acid pretreatment of lignocellulosic biomass (corn stover) followed by enzymatic hydrolysis (saccharification) of the remaining cellulose, followed by fermentation of the resulting glucose and xylose to ethanol. The pretreatment is described in more detail on pages 19 to 25 of the report. The pretreatment converts most of the hemicellulose carbohydrates in the feedstock to soluble sugars by hydrolysis reactions. Acetyl groups in the hemicellulose are liberated as acetic acid. It further reduces cellulose crystallinity and chain length. In the described design, hydrolysis reactions are catalyzed using dilute sulfuric acid and heat from steam. The pretreatment is said to be carried out in two stages.
The first stage is a horizontal screw-feed pretreatment reactor. The horizontal screw-feed pretreatment reactor comprises two plug screw feeders and acid for the pretreatment reaction is added at the discharge of each plug screw feeder. Transport conveyors combine feedstock from both plug screw feeders and deliver it to the pretreatment reactor. The reaction conditions in this horizontal screw-feed pretreatment reactor allegedly comprise a total solids loading of 30 wt %; a temperature of 158° C.; 18 mg acid/dry gram of biomass (where additional acid is added downstream of the pretreatment reactor); a pressure of 5.5 atm (81 psia); and a residence time of 5 minutes. The first stage pretreatment reactor is discharged to a stirred flash tank that is controlled to keep the temperature at 130° C.
In the second stage the slurry of the flash tank is forwarded to the secondary oligomer conversion stirred reaction vessel, where it is held at 130° C. for 20-30 minutes and additional 4.1 mg/g of sulfuric acid is added, bringing the total acid loading to 22.1 mg/g dry biomass.
In the second stage most of the xylose oligomers leaving the first stage are converted to monomeric xylose.
Hereafter the slurry is flash-cooled.
The horizontal reactor configuration for the first stage pretreatment reactor is said to be chosen because it permits tighter residence time distribution control. According to the report this is important to minimize “over-cooking” or “under-cooking” portions of the biomass, either of which would lower the overall yield.
The reactor system in the report of Humbrid et al is said to be constructed of carbon steel with all parts in contact with acid, clad in expensive Incoloy 825.
At larger capacities, however, the construction costs for a plant for the conversion of lignocellulosic material into biofuels and/or biochemicals become a major factor. A process as described by Humbrid et al, where large parts of the reactor system are clad in expensive Incoloy 825 would therefore be uneconomical, especially when scaled up.
It would be an advancement in the art to provide a cheaper, but still efficient process for the treatment of lignocellulosic material. Furthermore it would be an advancement in the art if such a process would allow for long residence times and/or high volumes to allow scaling up to larger capacities.